Nitrogen oxides (referred to hereinafter as "NOx") which are contained in various types of exhaust gas not only threaten the health of individuals but are in general environmentally undesirable, since they may cause acid rain or photochemical smog. Accordingly, the development of an effective means for exhaust gas treatment, specifically the development of an effective NOx abatement method, has been sought by those in the art.
Some processes for reducing the NOx content of exhaust gases using catalysts have been put into practice. As examples of such processes, there can be mentioned a ternary catalyst process for gas fueled automobiles and a selective catalytic reduction process using ammonia for exhaust gases which are discharged from heavy industrial installations comprising, for example, boilers.
JP-A-63-283727 and the like (the term "JP-A" as used herein signifies an "unexamined published Japanese patent application) describe a process for reducing NOx in exhaust gases to nitrogen, the exhaust gases containing hydrocarbons which comprises bringing the exhaust gases containing NOx into contact with a zeolite having dispersed therein various types of metals. Various metals have been added to such a zeolite, and the resulting product is utilized as the catalyst.
In the first process proposed above, i.e., in the ternary catalyst process, the hydrocarbon components and carbon monoxide initially present in the waste gas which is exhausted upon combustion of the fuel are converted to water and carbon dioxide with the aid of a catalyst while consuming the oxygen in the NOx to thereby reduce it to nitrogen. In this process, however, the combustion must be carefully controlled in that a stoichiometric amount of oxygen is supplied to the combustion system so that the oxygen content in the NOx compensates for the amount of oxygen consumed in the oxidation of the hydrocarbon components and the carbon monoxide. Thus, this process is, by principle, not applicable to combustion systems where excess oxygen is present, such as systems involving a diesel engine.
The second process, i.e., the selective catalytic reduction process using ammonia, requires a very large scale installation for the sake of safety, since the process handles toxic and inflammable ammonia under high pressure. Accordingly, the process is not applicable, from the technological viewpoint, to systems comprising movable exhaust gas sources.
The last process mentioned above, i.e., the process for removing NOx from a hydrocarbon-containing exhaust gas using a zeolite catalyst, is intended for application mainly to gas-fueled automobiles. Accordingly, the process is unsuitable for treating exhaust produced by combustion in a diesel engine. If the process were to be applied to diesel engines, the activity of the catalyst would be insufficient to achieve the desired level of NOx removal. In more detail, since metal components are present in the catalyst, the catalyst not only suffers degradation due to the sulfur oxides discharged from the diesel engine, but also loses activity due to aggregation or a like behavior of the metal components thereof. Thus, this process has not been put into practice for the removal of NOx from exhaust gases discharged from diesel engines.
In light of the above, the present inventors conducted intensive research to overcome the problems set forth above, and, as a result, found that the use of a particular catalyst removes the NOx components from exhaust gases at high efficiency without a loss of activity even in the presence of sulfur oxides. It was further found that this removal process can be conducted without producing harmful substances such as carbon monoxide. This is an aspect of the present invention which makes the process disclosed herein highly advantageous, since conventional processes frequently discharge such substances into the atmosphere, depending on the reaction conditions involved. The present invention was accomplished based on such findings.